The alkylation of isobutane with light olefins to produce alkylate gasoline has been known and industrially used for more than 80 years. Anhydrous hydrofluoric acid (HF) and concentrated sulfuric acid have been the traditional catalysts used in refineries for isobutane alkylation. In more recent times, ionic liquids such as, for instance: alkylpyridinium heptachlorodialuminate and alkylimidazolium heptachlorodialuminate, in the presence of traces of strong Brønsted acid promotors have been proven to be effective catalysts for isobutane alkylation to make alkylate gasoline.
Alkylation of heavier isoalkanes with the traditional alkylation catalysts (HF and sulfuric acid) to make distillate-range products can be challenged by accelerated catalyst passivation and comparatively low catalytic activity. Heavy alkylates are occasionally formed as a by-product in the traditional gasoline alkylation processes. While these heavy alkylates are in the distillate-range, they are typically not useful for diesel applications because of their highly branched nature and low cetane number. These heavy alkylates from isobutane alkylation are too light for lubricant uses, but even if base oil-range by-products could be synthesized using tractional alkylation catalysts, their high level of branching is likely to give low viscosity index (VI).
More recently, ionic liquid catalysts have proven effective for making alkylates in the distillate-range from heavier olefins and isoalkanes than the C3-C5 feedstocks traditionally used to make alkylate gasoline. By selecting the olefins and isoalkanes used in alkylation with ionic liquid catalysts it has been possible to make alkylate products with a sufficiently low amount of branching that have useful cetane numbers. However, until now the synthesis of isoalkanes in the base oil-range by alkylation of heavy feedstocks has not been demonstrated.
Farnesene is a C15 tetraolefin that exists in a number of different isomeric forms and may be prepared in a number of ways from different feed stocks. One of these isomers, beta-farnesene, is particularly interesting because it can be made by fermentation of sugar which is available from renewable sources in very large volumes at low costs. Consequently, beta-farnesene is a renewable material that potentially could be produced in large volumes at an attractive price. Beta-farnesene can be hydrogenated to make farnesane, which is a diesel-range product already marketed as a renewable diesel. Lubricants and base oils typically have higher value than diesel, but farnesane is too light a molecule for most lubricant applications and in order to bring it into the more valuable base oil boiling range it is necessary to increase the molecular weight.
Farnesene may be oligomerized in the presence of an oligomerization catalyst to make heavier products, but since farnesene is poly-olefinic such oligomerization will typically be accompanied by a high degree of internal cyclization forming highly cyclic products. In order to address this issue the farnesene may be hydrogenated to the corresponding mono-olefin, which may then be oligomerized to make the targeted base oil boiling range products with significantly less cyclization. Co-oligomerization with alpha-olefin co-monomers is known to improve the VI of the products and quite reasonable quality base oils are potentially available by this method (US20140221258A1).
However, since saturated farnesane is not reactive and the di-olefin made from farnesane tends to cyclize in the oligomerization reaction, the oligomerization of farnesene demands a high selectivity to the mono-olefin, which is inherently difficult to achieve. This weakness potentially causes significant base oil yield losses and there is therefore a need for an alternative approach.
Contrary to the selective hydrogenation of farnesene to the mono-olefin, the complete hydrogenation of farnesene to farnesane is easily accomplished in essentially quantitative yield. But until now, there has not been a good process for converting farnesane to hydrocarbons in the base oil boiling range.